Dished, annular, radio frequency absorber and method of manufacture



Feb. 19, 1963 w. J. DWYER $073,451

DISHED, ANNULAR, RADIO FREQUENCY ABSORBER AND METHOD OF MANUFACTUREFiled April 7, 1958 ates ate 3,78,4l Patented Feb. 19, lfifiii Thisinvention relates to an improved radio frequency absorber for absorbingstray, reflected, incoming signals in antenna and to an improved methodfor making the ante.

Resistor mats and resistor honeycombs now used in antenna systems aresometimes formed of absorbing resistive materials that are difficult tomechanically machine into the desired configuration.

it is the object of the invention to provide a resistor element forabsorbing stray reflected signals from the concave reflector of anantenna which can be first assembled into a laminated block and then canbe mechanically machined into dished, annular shape without shredding,tearing, chipping or breaking during the process.

Another object of the invention is to provide a dished, annular, radiofrequency absorber formed of flexible tapes having an electricallyresistive coating and bonded to strips of self supporting, curedplastic. The electrically resistive tape has a given resistive value anda given carhon-binder formulation to eliminate the possibility ofreflection back to the reflector.

A further object of the invention is to provide a radio frequencyabsorber with little or no absorption effect on incoming signals passingtherethrough but with substantially total absorption effect on straysignals angularly reflected back toward the element from a concave metalreflector.

Other objects and advantages of the invention will be apparent from theclaims, the description of the drawing and from the drawing in which:

EEG. i is a perspective view in section of a dished, annular, radiofrequency absorber constructed in accordance with the invention.

FIG. 2 is a front view thereof.

FIG. 3 is a diagrammatic exploded view of the device of this inventionin use in a typical antenna system and H68. 4 and 5 are diagrammaticviews of the steps in the method or" making the device.

As shown in 3 an incoming radio frequency signal is polarized as itpasses through the polarizing member 22 in the direction of the openheaded arrows, which denote the direction or" travel of the radiofrequency energy. The polarized signals then strike the concave metalmom er 3., vhich is a reflector and reflects the signals to the focusAll of the incoming signals are not reflected by the member 21 to thefocus 45, some of the signals, shown by closed arrow heads, beingreflected outwardly, with a loss of polarization, at various angles ofincidence. These stray reflected signals interfere with the incomingpolarized signals and the absorber 23 of this invention is designed toabsorb all such stray signals. The crystal l8 accepts the signals forthe control system of the missile in a well known manner.

The absorber 23 is of dished, annular configuration with an outer rimportion 24 of substantial depth for spacing purposes. The centralportion 25 is uniform in thickess and includes the central opening 26.The front peripheral face 27 and the rear face 28 are parallel andsmooth and the front recess 29 is also smooth faced.

Absorber 23 makes use of an electrically resistive tape material similarto the resistor material described in my United States Patent No.2,781,277 issued February 12,

2 1957. As described in that patent, an asbestos tape is coated with anelectrically resistive material such as carbon ground into a very finepowder, which is mixed with a binding agent such as silicone to form aliquid suspension. Solvents are driven off by an air-drying process andthe tape is then cured by heating in an oven at a temperature of about300 C. The resulting asbestos resistive tapes are designated 31 herein.The electrically resistive film applied to the asbestos backing tape inthis invention is preferably a mixture of epoxy resin, phenolic resin,carbon, graphite and butyl Cellosolve (Z-butoxy ethanol) as a solvent.It may be applied by spraying, in a series of layers, as in my abovepatent or otherwise, to accurately control and produce in the tape 31the resistive characteristics desired in an RF absorber for use at aspecified frequency.

A plurality of identical, thin, flexible, asbestos, electricallyresistive tapes 31 are laminated alternately between a plurality ofidentical, self supporting strips 32 of cured plastic or plastic foamwith the edges of the tapes and strips in front to back specialarrangement. The tapes and strips are bonded to each other by means oflayers of binder material 33 such as epoxy resin cured in an oven at C.for 20 minutes to form a unitary, laminated, block 34. The strips 32 arepreferably of rubber-polymer foam, commercially available as Cooperformula Hycar (butadiene co-polymer with acrylonitrile).

The resulting laminated block 34 is shown in FIG. 5 and, because of itsasbestos resistive tape and cured plastic laminations, can bemechanically machined into the desired form as shown diagrammatically inFIG. 6 such machining may be by lathing or drilling with a suitablemachine tool 35, the drill 36 forming the central opening 26 and thecutter 37 forming the tapered dished front recess 29 with its smoothface 33 and the faces 27 and 28. The materials do not chip, fracture ortear during the cutting or smoothing operations. Angularly spaced holes41 may be drilled in the block 34, around the inner periphcry of theannular absorber.

in the preferred embodiment illustrated, the annular absorber 23 isapproximately six inches in outside diameter and one and one half inchesinside diameter. The rim face 27 is spaced from the rear face 28 adistance of about .815 inch in depth and the rim face 27 is about .210inch wide. The central portion 25 of member 23 is about .310 inch indepth, or thickness, the plastic strips 32 are about .365 inch in widthand the asbestos resistive tapes 333. are about .010 inch in width.

As shown in FIG. 3 an absorber, or grating trap 23 is positioned in theantenna assembly between the polarizing member 22 and the concave metalreflector member 21, and parallel to the spaced parallel polarizingelements 39 of polarizing member 22, whereby if the polarizing lines arevertical the absorber lines are also vertical as shown.

The incoming radio frequency signals pass through polarizing member 22and pass through absorber 23 without substantial deterrence because theresistive tapes .31 are thin and edgewise to the signals, and the curedplastic strips 3-2 while relatively wide are dielectric and do notretard, or interfere with, the polarized signals. The concave metalmember 21 reflects the incoming signals to the focus 4d of the antennaand any unpolarized stray signals reflected outwardly enter the absorber23 to be totally absorbed without interfering with the incoming signals.

The asbestos resistive tape must be of a predetermined resistive valuefor example, 300 ohms per square and of a predetermined carbon binderformulation, for example, 'epoxy resin; phenolic resin; carbon;graphite; and butyl Cellosolve as a solvent to properly absorb thereflected radio frequency signals Without re-reflection. The carbonbinder material is applied in a relatively uniform thickness of fil-m tothe tape and the ohmic value of the resulting product is then measured.If the measurement indicates that the ohmic value per square is notcorrect for the particular frequency desired, the proportions of thecarbon binder formulation are changed to give either a lower or higherper square ohmic value until the desired value for example, 300 ohms persquare is obtained. Thereafter the tapes 31 may be accurately producedusing the formulation determined to be correct by the above trial anderror method.

The depth of the portions 24 and 25 of absorber 23 and the spacing ofthe resistive tapes 3 1 depend on the particular selected frequency ofthe incoming signal and can be calculated by one skilled in the art. H

Reference is made to pages 247248 of Radar Engineering by Donald Fink,published in 1947 by McGra-w- Hill Book Co. Inc. The position of theabsorber 2 3 in relation to the paraboloidal reflector 211 is criticalin that it be placed near the focus indicated at 45 but external to theface plane indicated at 44. The formula where p is the distance measuredalong, the axis from a point on the axis known as the focus to the curveshows that an incoming signal passing through the face plane 44,contacting the paraboloid 21 and reflected thereby will reflect towardthe focus 45, since the incoming signal is parallel to the x axis by wayof the polarizing segment 22. The terms x and y in the above formula arethe axes of the parabola with the x axis at right angles to the faceplane of the parabola and the y axis at right angles to the x axis andparallel to the face plane all as shown in FIG. 168, page 248 of theabove mentioned article.

Such R-F energy as is not absorbed at the focus 45 will be reflected inan outward direction from the paraboloid 211. This R-F energy isscattered energy and passes through the face plane 44 of the paraboloidat all angles of scatter. This scattered energy enters the absorber 23at all angles and due to'the end spacial arrangement of the resistancetapes 31 plus their depth, the scattered energy contacts the stripsbroadside at various angles and is absorbed. V

T he resistance value of the absorbing resistance material 31approximates that of the characteristic impedance of empty space whichis 377 ohms per unit square. This remains more or less constant for anyunit regardless of frequency and wave length.

The distance between resistance tapes 3 1 is determined by the fact thatall tapes must be placed directly behind the metal lines of thepolarizing segment 22. Therefore, since metal lines of the polarizingsegment are spaced according to the wave length at any given frequency,the resistance tapes are also spaced according to the wavelength at anygiven frequency. For example, the particular element described herein isdesigned to operate on a one quarter wavelength. Using the wavelengthformula where f-is' the frequency in cycles per second, 7\ is Wavelengthin feet if v is velocity in feet per second, the spacing of metalpolarizing lines for the unit operating at a specific frequency can bedetermined. The spacing of the metal grid lines in this unit at itsspecific frequency of operation turns out to be 0.125 inch on centers.The spacing of the tape resistors 31, and the thickness of the plasticstrips 32, is .365 inch as stated above because it is not necessary thatthere be a tape resistor in alignment with every metal line of the grid22.

I claim:

l. A dished, annular, radio frequency absorber adapted to be interposedbetween the polarizing grid and reflector of a microwave antenna, saidabsorber comprising a laminated, unitary, self supporting body ofdished, annular shape formed of spaced, parallel, electricallyresistive, thin, flexible, asbestos tapes of substantial depth, edgewisefacing, normal to, and external of the face plane of said reflector,said tapes being each bonded to and supported by a pair of relativelythick, parallel strips of dielectric plastic material, whereby polarizedincoming signals pass through said body with minor absorption but straysignals reflected from said reflector member are absorbed by theresistive tapes of said absorber. 2. Acombination as specified in claim1 wherein said dished, annular body includes a hat central portion ofuniform thickness and a rim portion having a rim face at a predetermineddistance from said central portion for spacing saidcentral portion fromthe face plane of said reflector member. H

3-. A combination as specified in claim 1 wherein said parallel stripsof dielectric plastic material are of cured rubber polymer plastic foamand the faces thereof, opposite the face plane of said reflector, aresmooth.

4. A combination as specified in claim 1 wherein said electricallyresistive tapes are of a predetermined carbonbinder formulation having apredetermined ohmic value per square bonded to a flexible asbestosbacking and said dielectric strips are of cured plastic foam.

'5. The method of making a. dished, annular, radio frequency absorberwhich comprises bonding a plurality of identical, relatively thick, selfsupporting strips of cured plastic alternately, in parallelism, with aplurality of identical, relatively thin, flexible tapes of asbestoshaving a coating of carbon particles bonded thereto, to form a laminatedblock and then mechanically machining said block into dishedconfiguration and drilling said block into annular configuration.

6. A dished, radio frequency absorber comprising a unitary,self-supporting body of dished, annular shape, said body being formed ofa plurality of alternate, thin, parallel, flexible, asbestos taperesistors of substantial depth and uniform width and relatively thick,parallel strips of self-supporting, dielectric, plastic material ofsubstantial depth, and uniform width, said tape resistors and saidplastic strips being adhesively united to each other to edgewise facethe front and back faces of said body and the thickness of said stripsbeing predetermined to space said tapes apart according to thewavelength of a predetermined frequency. a

7. The method of making a dished annular radio frequency absorber whichcomprises the steps of forming a plurality of identical strips of curedplastic foam, each having a width corresponding to the wavelength of aselected radio frequency, then bonding said identical plastic stripsalternately, in parallelism with a plurality of identical, flexible,asbestos tape resistors, each having a selected resistance value, toform a laminated block and then machining said block into dished,annular configuration by lathe cutting a rim therein and drilling anaxial bore therein.

References Cited in the file of this patent UNITED STATES PATENTS2,511,610 Wheeler June 13, 1950 2,610,250 Wheeler Sept. 9, 19522,724,112 Hepperle Nov. 15, 1955 2,736,895 Cochrane Feb. 28, 19562,822,539 McMillan Feb. 4, 1958 FOREIGN PATENTS 890,069 Germany Sept.17, 1953 OTHER REFERENCES NR'L Report 4137, D'arkfiex-A FibrousMicrowave Absorber, by H. A. Tanner et 211., April 20, 1953, NavalResearch Laboratory, Washington, DC.

6. A DISHED, RADIO FREQUENCY ABSORBER COMPRISING A UNITARY,SELF-SUPPORTING BODY OF DISHED, ANNULAR SHAPE, SAID BODY BEING FORMED OFA PLURALITY OF ALTERNATE, THIN, PARALLEL, FLEXIBLE, ASBESTOS TAPERESISTORS OF SUBSTANTIAL DEPTH AND UNIFORM WIDTH AND RELATIVELY THICK,PARALLEL STRIPS OF SELF-SUPPORTING, DIELECTRIC, PLASTIC MATERIAL OFSUBSTANTIAL DEPTH, AND UNIFORM WIDTH, SAID TAPE RESISTORS